Most implementations of a MIDI Thru port driver use either two NOT/inverter gates, or two NAND gates with inputs joined, so the UART signal coming from the optocoupler is buffered before connected to the 200 ohm resistor and the MIDI Thru socket.
I am planning to add a MIDI Thru port to my MB-SID but didn’t want to use a whole CMOS IC just for this. The MIDI Thru port only needs to sink or not sink the current, not actually output +5v. I thought a PNP transistor could do this nicely. So I put a BC557 with the base connected via a 10K resistor to pin 6 of the optocoupler (the UART signal, which is connected to the PIC’s RX pin), the emitter connected via a 200 ohm resistor to the MIDI socket (it’s PNP, so the emitter will sink the current when the base is low) and the collector to ground. I didn’t think I needed a resistor between collector and ground, since the current is already limited by the two 220 ohm resistors of the MIDI Thru socket.
This works for me, I quickly tested it with my PC and messages I send to the MIDI In/optocoupler get looped back through the MIDI Thru to my PC.
So… can anyone see if there’s a problem with this idea? It would be nice if someone with more electronics expertise could approve it ;D
